This invention relates generally to the continuous casting of continuous filaments of glassy metal alloys. Specifically, this invention relates to molten alloy flow control in the continuous casting process wherein the molten alloy is extruded through a nozzle onto a rotating quench surface.
Extruding a molten alloy through a nozzle onto a rotating quench surface is one of the several basic methods known for quenching a molten alloy to a glassy state in the form of a continuous filament. Examples are shown in U.S. Pat. No. 3,938,583 "Apparatus for Production of Continuous Metal Filaments", issued Feb. 17, 1976, to S. Kavesh, wherein extruded melt is continuously directed onto the peripheral surface of a rotating cylindrical quench wheel; in U.S. Pat. No. 3,881,541 "Continuous Casting of Narrow Filament Between Rotary Chill Surfaces", issued May 6, 1975, to J. Bedell, wherein extruded melt is directed into the nip of two counterrotating quench wheels; and in U.S. Pat. No. 3,939,900 "Apparatus for Continuous Casting Metal Filament on Interior of Chill Roll", issued Feb. 24, 1976, to D. Polk and J. Bedell, wherein extruded melt is directed onto the interior surface of a rotating annular quench wheel. Various nozzle configurations may be utilized, as for example in U.S. Pat. No. 3,976,117 "Converting Molten Metal into a Semi-Finished or Finished Product", issued Aug. 24, 1976 to E. Olsson, wherein the casting nozzle is in close proximity to the moving quench surface.
For commercial scale applications, an uninterrupted and continuous supply of molten alloy at the rotating quench surface is required so that filaments of indefinite length may be continuously cast. Conversely, nearly instantaneous on-off capability is desirable for emergency shutdown or other interruptions of operation. Additionally, pressure at the inlet of the extrusion nozzle must be controlled within narrow limits to maintain quality control of the transverse dimensions of the cast filament.
The degree of dimensional constancy along the length of the continuously cast filament is sensitive to variations in the physical characteristics of the stream of extruded melt impinging upon the rotating quench surface and therefore sensitive to extrusion pressure at the inlet of the extrusion nozzle. Dimensional control difficulties arise from at least two aspects of the operation. First, linear casting speeds are high, typically about 75 to 2100 meters per minute; and second, the thickness of the cast filament or strip is extremely small, typically about 50 microns or so. Glassy metal alloy filaments are necessarily thin due to heat transfer requirements, since extremely high quench rates, typically 10.sup.6 .degree. C. per second, are required to prevent crystallization in cooling the alloy from its melting temperature below its glass transition temperature.
The present invention utilizes an inverted pressure bell and associated feedback control means to maintain a nearly constant molten alloy level in the tundish as the molten alloy is extruded and therefore to provide a continuous supply of molten alloy at a substantially constant pressure at the extrusion nozzle inlet. Generally, the use of an inverted pressure bell for level control of molten metal within a crucible is known. Examples are given in U.S. Pat. No. 3,510,345 "Apparatus and Method for Automatically Controlling the Molten Metal Bath Level in a Metallurgical Process", issued May 5, 1970, to P. Marchant and in U.S. Pat. No. 3,522,836 "Method of Manufacturing Wire and the Like", issued Aug. 4, 1970, to D. King.
The present invention differs considerably from gross metallurgical processes as shown in Marchant's patent for the dip forming of steel wire or rod. In the production of continuous filaments of glassy metal alloys, successful production depends critically on the close control of the process variables, owing to the extremely high heat transfer rates required for glassy metal formation and the resulting extremely thin shapes of the cast filament.
The present invention also differs significantly from other methods for the continuous casting of glassy metal alloys that do not extrude a molten metal onto a moving quench surface. For example, in King's patent use of an inverted pressure bell is disclosed in a batch operation for maintaining the molten metal pressure at an orifice for continuously forming a meniscus, which is simultaneously swept away by a rotating quench surface (a wiping action). King's method involves casting rates much lower than those methods of concern in the present invention and therefore does not require a quick response controller. Also, King's method inherently provides for instantaneous on-off capability, controlled merely by stopping the rotation of the quench surface.